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We investigate the Γ 2 -X z1 intersubband dynamics in GaAs/AlAs quantum-well structures by time-resolved infrared pump and probe experiments. In the studied structure, the second r level in GaAs is nearly resonant to the first X z level in AlAs. We observe bi-exponentially decaying signals with a short time constant of the order of 1 ps and a longer time constant of about 7 ps at 10 K and 4 ps at 300 K respectively. The long-term decay represents the X z -Γ 2 transfer by elastic Γ-X scattering at low temperatures. At 300 K electron-LO phonon scattering accelerates the Γ-X transfer.

New electro-optical phenomena in quantum-well structures, i.e. modulation of the light absorption and birefringence due to carrier heating in a strong electric field, have been investigated. The effects have revealed different features in the three types of structures under investigation, namely: (1) well-doped n -type GaAs/AlGaAs multiple quantum wells, (2) barrier-doped n -type GaAs/AlGaAs superlattices and (3) barrier-doped p -type Ge/GeSi multiple quantum wells. Possible mechanisms of the phenomena have been discussed.

The Γ2–Xz1 intersubband dynamics in GaAs/AlAs quantum-well structures is investigated by time-resolved infrared pump and probe experiments. In the studied structure, the second Γ level in GaAs is nearly resonant to the first Xz level in AlAs, resulting in elastic Γ–X scattering. A biexponential behavior of ground-state recovery has been found with a time constant in the order of 1 ps for the Γ2–Γ1 relaxation and a time constant of 7 ps (at 10 K), which represents the Xz–Γ2 transfer.

Optical phenomena connected with intersubband electron transitions in specially designed structures with tunnel-coupled GaAs/AlGaAs quantum wells have been investigated. The observed birefringence and absorption modulation under longitudinal electric field applied parallel to the quantum layers have been explained by electron heating and electron transfer in real space.

Results of experimental investigations of the transformation of intraband light absorption spectra by the quantum-well electrons in a transverse electric field are presented. In addition to the familiar Stark effect, absorption oscillations in the photoionization band are detected. These oscillations are caused by electron transitions from the ground state in a quantum well to quasidiscrete levels arising in the continuum of states above the well due to the linear potential of the external electric field.

Electro-optic modulation in transverse (perpendicular to the layers) and longitudinal (parallel to the layers) electric fields was studied in simple rectangular and asymmetric tunnel-coupled GaAs/AlGaAs quantum wells (QW). The investigated phenomena can be used for fast amplitude and phase light modulation.

Optical phenomena in the mid-infrared range connected with interlevel and intersubband charge-carrier transitions in quantum dot and quantum well (QW) heterostructures under optical and electrical pumping were investigated. Spectra of interband photoluminescence are also presented. The existence of a metastable level in funnel-shaped QWs is experimentally confirmed. The intersubband transition dynamics in asymmetrical pairs of tunnel-coupled QWs was studied by means of pump-and-probe time-resolved spectroscopy. This paper was presented at the 3rd Russian Workshop on Nanophotonics, Nizhnii Novgorod, Russia, 26-29 March 2001.